This invention relates generally to synthetic layered structures composed of two or more thin solid films and more particularly, it relates to a method for selective intermixing of such layered structures by utilizing a laser or electron beam as the energy source.
By way of background, a particular class of synthetic layered structures of considerable importance is the semiconductor superlattice in which two semiconductor materials with different electronic properties are interleaved in thin layers either (1) by depositing sheets of two semiconducting materials in an alternating manner, or (2) by introducing impurities into layers of a single semiconducting material. The former is designated as a compositional or heterostructure superlattice, and the latter is designated as a doping superlattice. Thus, a compositional superlattice comprises a periodic array consisting of alternating layers of two different semiconductors. Each layer has a thickness in the range from a single atomic layer up to several hundred atomic layers. In a compositional superlattice, the two semiconductors are so chosen that their band gaps, i.e., the difference in energy between the materials' valence and conduction bands, are different.
In the literature, a structure composed of two thin films of different semiconductors is called a single heterostructure. A structure composed of a semiconductor film of lower band gap sandwiched between two semiconductor layers of larger band gap is referred to as a double heterostructure. When the middle layer is sufficiently thin, the structure is called a single quantum well (SQW). A periodic structure composed of alternating layers of two different semiconductors is sometimes also referred to as "multiple-quantum-wells" (MQW) depending on the thickness of the semiconductor layer with the larger band gap. For the purposes of this invention, the distinction between a superlattice and a MQW is not essential, and thus the term "superlattice" will be used henceforth for the sake of simplicity. As used herein, it is intended that the term "selective intermixing" includes also the case where all regions of a synthetic layered structure are intermixed as well as the case where only a limited portion thereof is intermixed.
Specifically, each layer of the semiconductor having the smaller band gap produces what is referred to as a potential well in either the conduction band or the valence band or in both. In terms of optical and electronic properties, important distinctions can be made between three different types of semiconductor superlattices, commonly referred to as types of I, I', and II depending on the relative alignment of the conduction and valence bands in the two semiconductors. However, for the purposes of this invention, such distinctions are not essential, and thus a further discussion thereof is not necessary. Inside each potential well, only certain energy states or levels are available to the confined carriers (electrons in the conduction-band or holes in the valence-band). The values of the energy levels available to the electrons can be selectively controlled by appropriate choice of semiconductor materials and the width of their layers. In this fashion, the electronic and/or optical properties of SQW's, or of compositional or doping superlattices can be tailored.
It is generally well known in the art that in a semiconductor multilayer structure, the semiconductor with the smaller band gap can be a material such as gallium arsenide (GaAs) and the one with the larger band gap can be a material such as aluminum gallium arsenide (Al.sub.x Ga.sub.1-x As) having a variable aluminum mole fraction x. Compositional superlattices and SQW's consisting of gallium arsenide and aluminum gallium arsenide are generally grown by metalorganic chemical vapor deposition (MO-CVD), molecular beam expitaxy (MBE), liquid phase epitaxy (LPE), or other suitable deposition techniques. The preferable techniques are generally considered to be the MO-CVD and MBE processes.
Since it is necessary to laterally modify the doping, mobility, band gap and refractive index of the epitaxially grown compound semiconductor layers (such as doping or compositional superlattices) for monolithic integration of all relevant optical components, there has arisen the need to perform selective intermixing of different semiconductor layers, typically of thickness ranging from about 5 .ANG. to about 5 microns, comprising device structures such as GaAs/Al.sub.x Ga.sub.1-x As superlattices and related heterostructures used for optoelectronics. Heretofore, in the prior art, such selective intermixing has been achieved by either localized diffusion or implantation of both donor or acceptor impurities.
In particular, there is disclosed in U.S. Pat. No. 4,378,255 a method by which a multilayer, III-V semiconductor structure can be disordered and shifted up in energy gap into single crystalline form by the expedient of a zinc diffusion. In particular, this U.S. Pat. No. 4,378,255 teaches that all or selected portions of a multilayer of either gallium arsenide/aluminum arsenide or gallium arsenide/aluminum gallium arsenide can be converted into a single crystal aluminum gallium arsenide having a higher energy gap than that of the original structure by the process of a zinc diffusion at low temperature. However, this prior art technique suffers from the disadvantage that substantial intermixing of the superlattice layers requires diffusion times on the order of hours. Furthermore, this prior art technique is unable to obtain such intermixing without the introduction of substantial impurity atom concentration into the superlattice materials.
Also recently reported in the literature is a related technique for the formation of Al.sub.x Ga.sub.1-x As alloy on a semi-insulating GaAs substrate by irradiating a two-layer structure of AlAs and GaAs with a high power continuous wave (cw) argon laser. This technique was reported by N.V. Joshi and J. Lehman in "Formation of Al.sub.x Ga.sub.1-x As Alloy on the Semi-insulating GaAs Substrate by Laser Beam Interaction", Materials Research Society Symposia Proceedings, Vol 51, p. 185-189 (1986). However, the reported quality of the alloyed material obtained by this method, based on a stationary or quasi-stationary cw beam, appears to be inferior and often leads to destruction of the material. No mention is made in this work of the use of pulsed or of rapidly scanned laser or electron beams with dwell times of a fraction of a second to grow alloys or to intermix relatively complex structures to obtain intermixed materials of high quality or final composite structures of high complexity.
It would therefore be desirable to provide an improved method for more rapid selective intermixing of multilayered materials which provides material of superior electronic and optical properties and which, in addition, avoids the necessity of introducing impurity atoms into the material as occurs in the method taught by U.S. Pat. No. 4,378,255. Moreover, in contrast to the work of Joshi and Lehman, it would be desirable to provide a new method that would be applicable to the intermixing of complex multilayered structures such as laser heterostructures and superlattices. It should be apparent that such an improvement would enhance the entire field of integrated optics/optical signal processing. Such improvements are achieved in the present invention by utilizing pulsed or rapidly scanned directed energy sources such as those derived from suitably chosen laser beams or electron beams for intermixing of multilayered structures.